Filter body for a soot filter

ABSTRACT

Filter body with internal cavities for use in particulate filters for internal combustion engines, which is preferably produced using micro-patterning processes employed in semiconductor technology, such as for example etching or patterned deposition. Integrated resistance heating can be realized by means of the choice of an electrically conductive material for the filter body.

The invention relates to a filter body for particulate filters ofinternal combustion engines.

In recent years, there has been a great increase in the number of dieselengines used as drive systems for motor vehicles in Europe. However,emission problems, as one of the main obstacles to further extending theuse of these units, have become a focal point of interest. In particularthe emission of fine particles which can reach the lungs and aretherefore harmful to health represents a challenge which has not yetbeen sufficiently resolved. The potential of particulates withdimensions of less than 10 nm to be harmful to health is especiallyhigh, on account of the ability of such particulates to reach the lungs.

Currently, various concepts are being pursued with a view to reducingthe number and maximum diameter of the particulates emitted into theoutside air. For example, DE 39 41 698 A1 proposes a particulate filterwhich has a sintered body as the filter body. Although these bodies areeasy to produce, their properties are unsatisfactory with regard totheir pore dimensions, and consequently it is impossible tosatisfactorily ensure that in particular extremely small particles arefiltered out of the exhaust gas.

Furthermore, as time moves on during operation, the pores inconventional particulate filters increasingly become blocked by carbonparticulates, so that the flow resistance in the filter rises andregeneration of the filter becomes necessary, typically by burning thecarbon particulate to form CO₂ at approx. 600° C. Since the exhaust-gastemperature of modern diesel units does not generally reach this level,either the filter body has to be heated or the combustion temperaturehas to be reduced by adding fuel or additives to the filter. Typicalmethods and apparatuses used for this purpose are described, forexample, in documents DE 4329558 A1, EP 661429 B1, EP 806553 A2 and DE4117148 C2.

It is an object of the invention to ensure effective exhaust-gaspurification while at the same time making it easy to regenerate theparticulate filter.

This object is achieved by the apparatuses having the features describedin claims 1, 12 and 14. The features described in the subclaimsrepresent advantageous refinements of the invention.

Accurate micro-patterning of the filter body is necessary if extremelysmall carbon particulates are to be filtered out of the exhaust-gasstream. In this context, it is particularly desirable for the shape,orientation and dimensions of the cavities in the filter body to beoptimized with a view to the requirements that apply for an effectivefiltering action. According to the invention, this is achieved by virtueof the fact that processes employed in semiconductor technology are usedto pattern the body. The use of these processes allows the definedcreation of patterns even in the nanometer range; nowadays, they areused on an industrial scale. This makes it possible to filter outparticulates with a diameter of less than 10 nm on account of theaccurate design of the filter body which satisfies the particularrequirements.

It is advantageous for the etching processes which are known fromsemiconductor technology to be used for the micro-patterning. Goodresults are achieved in particular by ICP (Inductively Coupled Plasma)etching or anodic etching. An overview of known etching processes usedfor micro-patterning in semiconductor technology is to be found, forexample, under Köhler, “Ätzverfahren f{dot over (u)}r die Mikrotechnik”[Etching processes used in micro-technology], Wiley-VCH 1998. Thedeposition of whiskers has also proven a suitable form ofmicro-patterning. Whiskers are thread-like crystals which are generallyin single-crystal form or at most are composed of a small number ofcrystallites.

The design options which are opened up by selecting the productionprocesses referred to above can advantageously be used for the definedmicro-patterning of the filter body. By way of example, the dimensionsof the webs, cavities or pores of the filter body can be optimized witha view to optimum particulate filtering.

It is particularly advantageous to reduce the pore dimensions in thefilter body in the direction of flow, in order on the one hand to keepthe flow resistance of the overall filter system at a low level and onthe other hand to produce defined purification stages for optimizing thefilter action.

It appears particularly practical in terms of manufacturing technologyto produce individual partial filter bodies with constant poredimensions and to then assemble these partial bodies to form the overallfilter body. Furthermore, this procedure allows individual filterelements to be exchanged if necessary, for example for maintenancepurposes. Furthermore, it is advantageous for the partial filter bodiesthemselves for the overall filter bodies to be produced by sintering orbonding stacks of individual wafers which have in each case already beenmicro-patterned.

It is also advantageous for the filter body to be designed as amonolithic block with a pore size which decreases continuously in thedirection of flow. In this case too, the internal geometry of the filterbody can be optimally matched to the requirements of a particulatefilter.

To ensure that the particulate filter can be regenerated, it isrecommended for the filter body as a whole or in part to be producedfrom electrically conductive materials or to be coated with materials ofthis type. This eliminates the need to install a separate resistanceheating means.

If a heating voltage is applied to the filter body, the heating currentflows through the whole of the filter body or large areas thereof, sothat the filter body is heated and thereby completely cleaned. No localaccumulations of carbon particulates, which have an adverse effect onthe performance of the filter, are left behind.

On account of the filter also being heatable, the regeneration isindependent of the inlet temperature of the exhaust gas fed into thefilter. This allows the location where the filter is installed in theexhaust system to be selected optimally in terms of the space availableand also allows the heating power to be matched to the current loadingstate of the filter.

The electrical conductivity of the filter material can be ensured, forexample, by using suitably doped silicon. The electrical properties ofthe filter body can be optimized with a view to achieving an optimumregeneration action when heated, by means of the doping profilesselected. It is particularly advantageous for the electrical propertiesof the regions which are exposed to the carbon particulates to aparticular extent to be selected to be such that the heating power is atits maximum in these regions; this minimizes the power consumption ofthe heating while still achieving an optimum effect.

Silicon, germanium and compounds or solid solutions thereof arerecommended for use as filter body material, on account of theirmechanical, chemical and electrical properties. In particular silicon ischemically inert and mechanically stable even at high temperatures. Thisensures a long surface life of the filter body over a large number ofregeneration cycles. Furthermore, semiconductor technology givesextensive experience of the micro-patterning of the abovementionedmaterials. Of course, it is also possible to use other substances whichcan be micro-patterned by means of the processes described above.

A further advantageous configuration of the invention consists in thefilter body being completely or partially internally coated withelements from the platinum metal or rare earth groups, thereby achievinga catalyst effect. The reaction temperature at which carbon particulatesburn to form CO₂ is reduced as a result, and less heating power isrequired to initiate the oxidation reaction.

It is also proven appropriate, when using a silicon filter body, tooxidize the silicon in a controlled way and thereby, by means of thequartz layer which is formed, to make it inert with respect to all thecombustion products which are present.

A filter element which can be produced at low cost from sintered siliconparticles, for example, can advantageously be used as an upstream filterfor filtering out large carbon particulates (>100 nm). The production ofthe associated filter body is described in U.S. Pat. No. 4,767,585. Themean pore dimension is set by the particulate size of the startingmaterial. A catalytic coating of platinum, for example, produced byelectroplating preferably takes place after the sintering of the shapedbody. The electrical resistance required for direct heating of thefilter element is set by doping of the silicon starting material and/orby means of the coating formed by electroplating and the geometricshape.

The filter body described above is easy to integrate to form aparticulate filter. By way of example, the geometry of the filter bodycan be selected in such a way that it can easily be introduced into ahousing having the dimensions of conventional particulate filters whichare already in use in vehicles. Therefore, conventionally equippedvehicles can be retrofitted without any further structural measureshaving to be carried out on the vehicle.

Furthermore, it is advantageous for the filter body described also to befitted in new motor vehicles.

FIG. 1 illustrates an example of a structure of a particulate filterusing filter bodies according to the invention.

The illustration shows a longitudinal section through a particulatefilter which has three partial filter bodies with different dimensionsof the internal cavities. The partial filter bodies 3 are arrangedstacked in the housing 1 of the particulate filter. This ensures thatindividual partial filter bodies can be exchanged for maintenance work.The decrease in the dimensions of the internal cavities of individualpartial filter bodies in the direction of gas flow, indicated by arrow2, is diagrammatically illustrated. This ensures that the size ofparticulates filtered out decreases in the direction of flow and thefine pores or passages at the end of the filter do not prematurelybecome blocked by carbon particulates. To ensure direct heatability ofthe filter body, the latter is contact-connected by means of the currentterminals 4 passed through the housing. When necessary, the particulatefilter illustrated can be heated in a simple way by applying a voltageto the current terminals and can thereby be regenerated by oxidation ofthe carbon particulates to form CO₂, which is subsequently dischargedfrom the filter in the gas phase.

1-14. (canceled)
 15. A filter body for use in a particulate filter foran internal combustion engine, the filter body having internal cavities,formed by using a semiconductor technology type micro-patterningprocess.
 16. The filter body of claim 15 wherein the micro-patterningprocess comprises an etching process.
 17. The filter body of claim 15wherein the micro-patterning process comprises a patterned depositionprocess.
 18. The filter body of claim 15 wherein the micro-patterningprocess comprises a combination of an etching process and a patterneddeposition process.
 19. The filter body of claim 15 wherein the internalcavities are of differing dimensions.
 20. The filter body of claim 19wherein the dimensions of the cavities decrease in a direction of flow.21. The filter body of claim 20 wherein the dimensions of the cavitiesremain constant for pre-selected parts of the filter body.
 22. Thefilter body of claim 20 wherein the dimensions of the cavities decreaseconstantly in the direction of flow.
 23. The filter body of claim 15wherein the filter body at least partially comprises electricallyconductive material.
 24. The filter body of claim 23 wherein theelectrically conductive material forms regions of differing specificconductivity.
 25. The filter body of claim 15 wherein the filter body atleast in part comprises a material selected from a group consisting ofsilicon, germanium, a silicon compound, a germanium compound and a solidsolution thereof.
 26. The filter body of claim 15 wherein at least partof the filter body has a catalytically active coating.
 27. The filterbody of claim 15 wherein at least part of the filter body has an oxidelayer.
 28. A particulate filter for a motor vehicle, comprising ahousing with at least one gas inlet and at least one gas outlet, and afilter body in the housing, the filter body being formed by using asemiconductor technology type micro-patterning process.
 29. Theparticulate filter of claim 28 further comprising at least one filterbody made from a sintered material.
 30. A vehicle including aparticulate filter comprising: a housing with at least one gas inlet andat least one gas outlet, and a filter body in the housing, the filterbody being formed by using a semiconductor technology typemicro-patterning process.